Stepfather is doing some remodeling at his dealership. I convinced him to keep the fluorescent fixtures rather than convert to LED, but I'm running into some problems.

1) A pair of F40T12 wrap fixtures. The old GE tubes would flicker for about an hour before finally reaching full brightness. Finally one of the fixtures wouldn't light. These use junky looking ballasts from a company I've never heard of. I put brand new Sylvania F40T12/CW in and it seemed to work when turned on even if it still took a while to reach full brightness (I figured they needed to burn in).

I came over today and after being on a while they were doing some weird kind of flickering. Like only half of three out of the four tubes was flickering, and the flickering was more like bands or swirls brightness traveling down the tubes in waves. What's going on and how to I fix it?

2) A pair of single light F96T12. The old ballasts were Advance and built like a tank, nothing wrong with them but ny stepfather was "I hate that hum, I don't use them because of that" and "There must be something wrong with the 'transformer' if they hum like that". So OK, I didn't want to but felt I had to replace it, so I got an Osram Instant Start ballast. Since it can drive two lamps I was going to use one lamp for both fixtures, but the wires won't reach the ends of both lamps. Will standard THHN wire (I think it's rated for 600 volts) or the old wire to the lampholders whatever it is handle the starting voltage? Do I just wire it to either side of the old lampholders, or does it have to go to both sides?

That swirling sounds like just what new lamps do sometimes. It should go away after some time (I think typical burn-in is 100h) it may be annoying but harmless.

As for the second question I have no answer since we dont have instant start ballast here nor F96T12 lamps.

1)
That swirling uses to be quite common with older lamps, it uses to disappear one the lamps fully warm up.

2)
I assume the instant start ballast is some electronic type.
These are designed for certain maximum capacitance on the wires and usually can not tolerate much more. The problems may be either just an excessive electromagnetic radiation (so it would act as an undesired radio transmitter, wiping radio signal all around), or it may interfere with the built in protection feature (it wont shut down bad lamp), quite likely resulting into toasted ballast.
Usually the wires, which are originally with the ballast, are of the maximum allowed length, so beter to not extend it any further.

Part 1 sounds like the swirling that occasionally happens with new tubes. It should go away, and turning the light off for a few seconds and then back on usually fixes it.

Part 2 You will need to rewire the fixtures; the existing wire should handle the voltage - it is usually rated 600 / 1,000 volt. If the fixtures are mounted end to end, it should be okay to extend the ballast wires a little bit, just use only as much as you need. I would run the red wire to the center sockets and the blues to the far sockets.

I guess I'm kind of confused how the original slimline circuit is supposed to work. The lamp only has one pin on each end, but one of the sockets has two wires, one of them going to neutral and one going to the ballast. The new instant start ballast I want one wire to go to each lampholder, to both sides?

The lamp only has one pin on each end, but one of the sockets has two wires, one of them going to neutral and one going to the ballast.

That serves to cut power to the ballast when the lamp is missing. It assures safe handling of lamps when re-lamping on live circuit. It is usually on the fixed lamp holder. Otherwise, when inserting into the plunger end, it makes the other end of the lamp live, and could result in electrical shock. This assures the ballast will only operate when (both) lamp(s) are properly seated in the holders. I hope this makes sense ; I'm just about ready for bed...

I don't know why the electronic ballast does not need that cut-out feature.

I don't know why the electronic ballast does not need that cut-out feature.

Electronic detect the lamp hasn't ignited properly, so shut down. To restart, they then need power to be cycled.

So when are you messing up with the lamps, the ballast detects that open circuit as a lamp failure, so does not generate any voltage capable of igniting the lamp anymore, so even when you stick the lamp into the normally hot end, there is already no high voltage.
Implementing the cutout as it was in magnetic is very difficult with electronic, as there the lamp circuit is on different potential than Neutral, so the wire can not be shared the same way as with the magnetic. The electronic would need an isolated switch (operated by inserting the cold lamp end) to get the same behavior as the traditional circuit, that means one socket would actually need three wires: One for lamp connection, other two for the switch (cutting power to the ballast).

Yep, sol is right; the magnetic ballast used the socket as a disconnect.

Most electronic instant start ballasts (at least in the U.S.) will continue to output high voltage when the lamps are not installed, so extra caution must be used if re-lamping with the power on.

The issue with the wraps is probably the ballast grossly under driving the tubes either by design or because of a failing capacitor. New lamps will mask the problems for a while. Replacing the ballast is the only true fix though.

The issue with the wraps is probably the ballast grossly under driving the tubes either by design or because of a failing capacitor. New lamps will mask the problems for a while. Replacing the ballast is the only true fix though.

I'm seeing this regularly on full power preheat ballasts (F36T8, 0.43A reactor ballast on 230V, electronic starters giving good preheat), the only thing there is, those lamps are usually fired after few week rest during the summer, or even half a year rest over the winter season. After few days of regular use (operating about 7..8 hours per day, with two starts), the effect is gradually disappearing though.
So if the fixture is resting in cold and only occassionally used, it would be normal even with a full power ballast.

Sylvania = Swirlvania.  A problem with then when new. They will quit that after some hours are on. 

Just turn the power off, then back on, and the spiralization will cease. 

Is the fixture mounted near a cold draft such as a vent? I have seen slimlines do this. Fluorescent tubes do not like cold air.